Ubiquitous to most cells, enzyme systems have evolved for protection against oxidative stress. One system, which includes suproxide dismutase (SOD), acts to remove molecular oxygen and its univalent reductants, thereby protecting the cell from the harmful effects of these reactive oxygen species. Although an obligate anaerobe, the putative periodontopathogen Porphyromonas gingivalis exhibits considerable aerotolerance and it has been suggested that SOD may play a key role in this activity. In addition, SOD may act to counter host defense free oxygen radical activity. One recent study demonstrated that insertional inactivation of the sod gene in P. gingivalis ATCC 33277 resulted in a rapid loss of viability and cell Iysis. By allelic exchange mutagenesis, we constructed in the laboratory of Dr. Howard Kuramitsu, an isogenic P. gingivalis 381 mutant defective in the sod gene. As confirmed by Southern blot analysis, a significant portion of the sod gene has been deleted and replaced by a gene cassette conferring erythromycin resistance. Growth of the mutant under aerobic culture conditions resulted in cell Iysis, whereas wild type P. gingivalis 381 was not affected. Furthermore, cell viability of the mutant was diminished by greater than four orders of magnitude within three hours. These results support earlier finding that SOD is essential for aerobic tolerance in P. gingivalis. Ongoing studies to determine the role of this gene in evading host response mechanisms include neutrophil killing assays and implantation in animal model systems. In addition, the effects of the host cell environment on SOD expression is under investigation utilizing sod-reporter gene fusions. It is anticipated that these studies will yield novel information regarding the role of this gene in P.